Here is an article from Google on the origins of voodoo -- one of the better ones.
The actual religion of Voodoo, or “Voudon”, originated from the ancient practices of Africa. Voodoo came about most likely in Santo Domingo (modern day Haiti) where slaves devoted rituals to the power of nature and the spirits of the dead. The term “voodoo” was probably adapted from the African Fon spirit, “vodu”.
For many enslaved Africans, such spiritual traditions provided a means of emotional and spiritual resistance to the hardships of life. In time, slaves from the Caribbean were brought to New Orleans and they brought Voodoo with them.
The first reference to Voodoo in official documents came in 1782 during the Spanish regime in New Orleans. In a document which tells of imports to the colony, Governor Galvez states "these Negroes are too much given to voodooism and make the lives of the citizens unsafe". He made an attempt to ban the importation of slaves for a brief time... and its likely he felt he had a good reason.
In both 1791 and 1804, a series of slave revolts rocked Haiti. The revolts were based around the practice of Voodoo and they ended with the French being expelled from the island. Many of the French were able to escape to New Orleans and many of them brought their slaves with them. Now, New Orleans had not only additional Voodoo practicing slaves, but rebellious ones as well.
Many of the Catholic saints would become “stand-in’s” for important Voodoo deities and if you go into a Voodoo shop today, you will see statues, candles and icons depicting various Catholic images. There are in fact, Voodoo symbols as well.
Soon after the introduction of the African slaves to New Orleans, Voodoo began to play a major part in the traditions, and fears, of the general populace. It was not long before the white colonists also began to hear of it and to feel its power. By the end of the century, Voodoo was firmly entrenched in the culture of New Orleans.
The religion was practised by the slaves and the free blacks as well and so strong was the power held by the upper echelons of the religion that they could entice their followers to any crime, and any deed. Whether or not these priests held supernatural power or not, the subtle powers of suggestion and of secret drugs made Voodoo a force to be reckoned with. Masters felt the taste of poison in their food, women and men the taste of lust with a handful of powder... and even death was held in check by the use of "zombie" drugs. There was no denying that Voodoo was real, and powerful, and even today, it is widely practised in the Caribbean islands.... and even in New Orleans.